1. Field of the Invention
The subject invention is directed to systems for extruding thermoplastic shapes and, more specifically, systems and methods for the replacement of calibrators in plastic extrusion systems.
2. Description of the Prior Art
Various types of apparatus and methods for shaping materials according to selected profiles have been developed. These methods vary depending on the properties of the materials as well as other factors. Examples include aluminum extrusion and paltrusion of resin impregnated fibers as described in U. S. Pat. No. 5,116,450.
In the case of thermoplastic materials such as vinyl plastic, the material is shaped by being heated and extruded through a die. Basically, thermoplastic resins are fed into a heating element. The heated resin exits the heating element and is passed through a die of predetermined shape. The extruded length is drawn by a puller that maintains predetermined tension on the material length. The drawn length of material is then cooled and cut to a given length. As the material exits the die; it has sufficient plasticity that steps must be taken to maintain the extruded shape while heat is removed from the material until it attains sufficient rigidity.
Various mechanisms have been developed in the prior art to maintain this extruded shape. Frequently, calibrators such as described in U.S. Pat. Nos. 4,468,369 and 4,120,926 are used for this purpose. These devices selectively apply vacuum to the extrusion to control its shape. The interior walls of the calibrator provide additional sizing. At the same time, cooling water is circulated to internal passages in the calibrator to provide temperature control of the calibrators. Heat is thereby conducted away from the extruded material to increase its rigidity.
Frequently, a plurality of calibrators are used, due to limitations on deformation in the plastic material through any single calibrator. These are arrayed in linear fashion such that the extruded material serially passes through each calibrator and is progressively shaped and solidified as it passes through each calibrator.
It has been recognized in the prior art that each of the calibrators should be aligned with the longitudinal axis of the extruded material to reduce the strain on the material. Non-alignment of the calibrators tends to cause increased strain on that portion of the material that is in tension. This tended to slow the rate of manufacture and also adversely affected product quality. In some cases, this could result in broken or poorly shaped lengths. In the prior art, the narrow alignment tolerances in combination with the multiple connections required for vacuum and cooling water lines has required substantial set-up times when there is a changeover to a different extruded profile. During this set-up time, the extrusion line could not be productively used adding to the difficulty and cost of the manufacturing process.
The problems resulting from calibrator alignment requirements were recognized in the prior art as mentioned, for example, in U.S. Pat. Nos. 5,316,459 and 5,288,218. Attempts to address this problem generally involved incorporating all of the calibrator profiles in a single body as shown in U.S. Pat. No. 4,401,424. However, this required the redesign and manufacture of all existing calibrators.
Various schemes for pre-alignment of existing calibrators have been proposed from time-to-time. However, these have generally not been workable in practice because those pre-alignment techniques did not allow for differences in thermal expansion rates between the different components of the extrusion system. That is, the calibrators, which are typically made of aluminum, would vary in size in response to a given temperature change. The expansion rate of the calibrators was significantly higher than the rate of extrusion table and other system elements that were made of steel. This, in combination with the requirement for close alignment tolerances, created difficulty with certain pre-alignment techniques. For example, locator pins could not be used because the pins would register with the locator holes only within a narrow band of temperatures. If the tolerances in the pin-hole combinations were increased, the alignment tolerances for the calibrators would no longer be met.
Accordingly, there was a need in the prior art for a pre-alignment system and method that would meet tolerance limits for alignment of the calibrators and still be operable over a relatively wide band of temperatures.